About the Introduction of Passive System Unavailability in an Accident Sequence

Passive safety systems designed for new advanced Light Water Reactors (LWR) are devoted to plant safety, acting as safety features both to stop the progress and mitigate the consequences of an accident. This implies the evaluation of their unavailability within the plant probabilistic safety studies, with regard to the accident sequence definition and assessment.Classically in PSA (Probabilistic Safety Assessment) studies accident scenarios are modelled through the Fault Tree (FT) and Event Tree (ET) technique, which allow identifying all the different chains of accident sequences deriving from the initiating events, through a combination of safety system success and failure.The main issue concerns the introduction of a passive system in an accident scenario defined by the event tree model in the fashion of an active system or a human action. The reason is that its reliability figure depends more on the phenomenological nature of occurrence of the failure modes rather than on the classical component mechanical and electrical faults. This makes the relative assessment process different as regards the system model commonly adopted in the fault tree approach.